The invention is directed to protective coatings for turbine combustion components. In particular, the invention is directed to protective coatings for outer surfaces of turbine combustion components.
The efficiency of turbines, for example gas turbines, is increased as the firing temperature, otherwise known as the working temperature, of the turbine increases. This increase in temperature will result in at least an increase in power with the use of the same, if not less, fuel in a turbine. Thus, it is desirable to raise the firing temperature of a turbine to increase the efficiency.
However, as the firing temperature of gas turbines rises, the metal temperature of the combustion components, including but not limited to combustion liners and transition pieces otherwise known as ducts, increases. A combustion liner is incorporated into a turbine, and defines, in part with a transition piece or duct, an area for a flame to burn fuel, especially residual fuels. A transition piece is in communication with a combustion liner, and further defines, with the combustion liner, a hot gas path for the turbine. However, the increase in firing temperature undesirably reduces rupture and fatigue properties of turbine combustion components, such as combustion liners and ducts. Further, this increase in firing temperature also severely and undesirably degrades oxidation or hot corrosion resistance of combustion components, such as combustion liners and ducts.
Turbine combustion components, such as combustion liners and ducts, are often formed of heat resistant materials. Turbine combustion components, such as combustion liners and ducts, are often coated on inner surfaces with other heat resistant materials. Combustion liners are formed, for example from wrought superalloys, such as but not limited to, Hastelloy alloys, Nimonic alloys, Inconel alloys, and similar alloys. These superalloys do not possess a desirable oxidation resistance at high temperatures, for example at temperatures greater than about 1500xc2x0 F. Therefore, to reduce the liner temperature and to provide oxidation and corrosion protection against a hot flame inside the combustion liner, a heat resistant coating, such as but not limited to, a bond coat and thermal barrier coating (TBC), are often applied on an inner surface, otherwise known as a hot side. For example, a bond coat and TBC are often utilized in aircraft engine and land based gas turbines.
As flame temperatures rise with a rise in firing temperatures, the outer surface of the combustion liner and the transition duct, otherwise known as a cold side of a combustion liner, often reaches very high temperatures. At the very high temperatures, oxidation and nitridation, especially when assisted by working stresses and thermal cycling, become a severe problem, especially at welds and their heat affected zones (HAZ) in combustion liners and ducts.
FIGS. 1-6 are microphotographs that illustrate at least one of severe grain boundary oxidation and nitridation in combustion liners, which were subjected to very high temperatures. The combustion liner is formed of Hastelloy X. The microphotographs illustrate the crack initiation and propagation along grain boundaries on the outer surface, otherwise known as the cold side, of the combustion liner. In FIGS. 1-4, cracks were initiated at heavily oxidized grain boundaries on outer surfaces of the combustion liners. Whereas in FIGS. 5 and 6, grain boundaries on outer surfaces of the combustion liners were heavily nitrided. Near the outer surfaces, cracks were initiated by oxidation of the nitrides along grain boundaries. The cracks then propagated intergranularly inwardly from the outer surfaces through the combustion liners. Therefore, oxidation and nitridation at outer surfaces of combustion liners are undesirable, and become a life limiting factor. Hot corrosion may also cause failure of a turbine combustion component.
FIG. 1 is a top view of a turbine combustion component at 200 times magnification and FIG. 2 is a bottom view of a turbine combustion component at 500 times magnification, where the cracks are visible due in part to a Kallings etching. FIG. 3 is a top view at 500 times magnification and FIG. 4 is bottom view at 1000 times magnification of a turbine combustion component, where cracks are visible again due in part to a Kallings etching. Both FIG. 3 and FIG. 4 are illustrated at locations outside a heat affected zone of a weld joint. FIG. 5 is at 500 times magnification and FIG. 6 is at 250 times magnification of a turbine combustion component. Both FIG. 5 and FIG. 6 illustrate blocky nitride phases mostly along grain boundaries near the outer surface of a combustion liner. Adjacent to the outer surface of a turbine combustion component, the nitride phase was oxidized to form cracks.
It has been proposed to solve problems associated with combustion liner cracking by reducing the firing temperature in a turbine or by redesigning the combustion liner. While both methods may appear to temporarily solve the problem, it has been discovered that both proposed solutions, reducing a firing temperature in the turbine and redesigning the combustion liner, sacrifice turbine efficiency. Further, both proposed require large amounts of re-work. The sacrifice of efficiency and the large amounts of re-work are, of course, undesirable.
Therefore, it is desirable to provide a turbine combustion component construction that overcomes the above noted, and other, deficiencies in the related art.
It is therefore desirable to provide a protective coating on an outer surface, otherwise known as a non-flame side, of a turbine combustion component, for example but not limited to a combustion liner and a duct.
Further, it is desirable to provide a turbine combustion system comprising at least one turbine combustion component, where the at least one turbine combustion component comprises an inner surface that defines a hot flame path area and an outer surface. A protective coating is positioned on the outer surface of at least one turbine combustion component to improve at least one of oxidation, nitridation and hot corrosion resistance of the at least one turbine combustion component.
These and other aspects, advantages and salient features of the invention will become apparent from the following 5 detailed description, which, when taken in conjunction with the annexed drawings, disclose the invention.